1. Field of the Invention
The present invention relates generally to wireless communications systems and networks and, more particularly, to improved timekeeping and synchronization of wireless communications systems and networks.
2. Description of the Related Art
Wireless communications systems and networks are rapidly advancing in capability and spreading in use. An example of a type of wireless communications system are cellular telephone systems which are commonplace everywhere in the world. These, and other, wireless networks (e.g. GSM, TDMA, CDMA) require accurate timing information to operate and to support new applications which are being developed. The accurate timing information may be needed in both the networks and the handsets (herein generically referred to as “mobile units”). Examples of network operations that could be assisted by accurate timing information include delivery of accurate time of day information to mobile units and synchronization of network timing—e.g. in base stations—to a common absolute time reference, which may be useful or even essential to some wireless technologies.
Wireless communications systems, and the mobile units they support, may also require precise timing information to allow accurate positioning of mobile units in the network with certain positioning technologies. Many countries, such as the United States (US), are beginning to require that network operators be able to quickly and accurately pinpoint the location of cellular telephones and devices in their networks. In particular, starting in October 2001, the US's Federal Communications Commission is requiring that mobile telephones sold in the US must be capable of being located with high probability to within 50 or 100 meters (depending on whether the positioning method is considered to be mobile unit or network centric, respectively) when the user places an emergency 911 call. Further, many applications, such as maps, direction finding, fleet management, child/spouse location, are being developed for mobile units which require accurate positioning information. Accurate timing information is required by certain positioning technologies to support such applications and positioning requirements.
Wireless communications systems and networks may also require precise timekeeping to achieve accurate timing in mobile units within the network. This timing information may be important to users of the units and to applications supported within the units and may also be used to improve operation of the units in conditions of faint signal strength.
A number of different techniques have been suggested to achieve precision timing in wireless communications systems and networks. One technique is to install Global Positioning System (GPS) clocks at each base station within the network. GPS clocks are GPS receivers that, given their precise geographical location, resolve GPS-time. GPS clocks allow each base station to benefit from the highly accurate clock signals received from the GPS constellation of satellites. In one application, the transmission timing of each base station may be synchronized to absolute GPS time. In such a case, absolute GPS time would be directly available to mobile units within the network by observing the transmission from any local base station. In another application, the knowledge of absolute GPS time by a base station could be used to accurately timestamp certain events. For example, certain uplink network centric positioning methods operate by means of timestamping the arrival of signals from a mobile unit at a number of different geographically dispersed base stations. The time differences between the arrival of a signal from the mobile unit at different base stations, which may be obtained from the timestamping information, may then be used to calculate the geographic position of the mobile unit when the known geographical positions of the base stations are also taken into account. Unfortunately, however, installation of GPS clocks at each base station increases the cost and complexity of building and maintaining a wireless network.
Another technique to achieve precision timing is to integrate a number of location measurement units (LMUs) into the network. LMUs are devices that may be used to obtain accurate information on relative or absolute timing. To obtain relative timing, a LMU may measure the arrival of transmission timing information from pairs of nearby base stations. In particular, a LMU may measure the apparent difference in the timing information transmitted by each base station in any pair (e.g. by measuring the difference in the arrival time at the LMU of some common timing related marker in the transmission signals from each base station).
Because the timing information from each base station reaches the LMU after a certain delay proportional to the distance between the base station and the LMU, the apparent difference in timing between two base stations, as measured by the LMU, needs to be corrected by adding or subtracting (depending on whether time advance or time lag is to be treated positively) the difference between the respective propagation delays to obtain the real difference in timing between the base stations. The difference in the propagation delays can generally be known very accurately from the geographic positions of these entities. The correction may be performed by the LMU or by some central network entity that receives measurements from the LMU. The end result is knowledge by the network of the relative differences in the local transmission timings of the base stations. These timing differences may then be employed to assist with certain positioning methods, such as the E-OTD position method used in GSM.
LMUs may also be employed to measure the association between the local transmission timing of any base station and some absolute time reference like GPS time. In this case, the apparent time difference observed at the LMU must also be corrected by taking account of the propagation delays between the LMU and the base station and between the LMU and any GPS satellite source. The ensuing corrected time associations, which are now with respect to an absolute time source, may be used to support many applications including a number of geographical position methods like E-OTD and various enhancements of GPS.
Unfortunately, these LMU based approaches require the use of additional equipment and integration, thereby increasing the overall cost and complexity of building and maintaining the wireless network. The cost of implementing and deploying such LMUs could be a significant proportion of the overall infrastructure cost in a wireless network.
It would be desirable to provide a system and method for timekeeping and synchronization of wireless communications systems and networks which overcomes the drawbacks of previous systems and methods. It would be desirable to provide a system and methods which allow accurate absolute and relative timing in wireless networks. It would further be desirable to provide a system and methods which allow the accurate timing to be achieved without expensive or additional network equipment, such as LMUs and/or GPS clocks integrated into base station equipment.